Vacuum pump

ABSTRACT

A vacuum pump may include a regenerative pumping mechanism and a drag pumping mechanism. The regenerative pumping mechanism may include a generally disc-shaped member mounted on an axial shaft for rotation relative to a stator of the regenerative pumping mechanism. The drag pumping mechanism may include a generally cylindrical-shaped member mounted at an outer circumferential portion of the disc-shaped member for rotation about the axial shaft relative to a stator of the drag pumping mechanism.

This application is a national stage entry under 35 U.S.C. §371 ofInternational Application No. PCT/GB2013/050363, filed Feb. 15, 2013,the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a vacuum pump comprising regenerativepumping mechanism and a drag pumping mechanism.

BACKGROUND

WO2010/133866 is an earlier application of the present applicant. Theearlier application discloses a vacuum pump comprising a regenerativepumping mechanism and a drag pumping mechanism. The regenerative pumpingmechanism and the drag pumping mechanism comprise respective generallydisc-shaped members which are mounted to an axial drive shaft at axiallyspaced locations on the drive shaft. Fluid flow paths are generated forconveying fluid through the drag pumping mechanism and then radiallyoutwardly through the regenerative pumping stage towards the outlet.Fluid conveyed through the drag pumping mechanism passes over a firstsurface of the disc-shaped rotor and then over a second surface of thedisc-shaped rotor. The pressure of the fluid over the two surfaces isdifferent (the pressure over the first surface is lower than thepressure over the second surface) and gives rise to different forces.The disc-shaped rotor is not therefore balanced and this can give riseto problems in certain pumping conditions.

SUMMARY

The present invention seeks to provide an improved vacuum pump.

The present invention provides a vacuum pump comprising a regenerativepumping mechanism and a drag pumping mechanism, the regenerative pumpingmechanism comprising a generally disc-shaped member mounted on an axialshaft for rotation relative to a stator of the regenerative pumpingmechanism, the drag pumping mechanism comprising a generallycylindrical-shaped member mounted at an outer circumferential portion ofthe disc-shaped member for rotation about the axial shaft relative to astator of the drag pumping mechanism, rotation of the disc-shaped memberand the cylindrical-shaped member by the axial shaft causing fluid toflow radially inwardly in series through the drag pumping mechanism andthe regenerative pumping mechanism.

Other preferred and/or optional aspects of the invention are defined inthe accompanying claims.

BRIEF DESCRIPTION OF DRAWINGS

In order that the present invention may be well understood, anembodiment thereof, which is given by way of example only, will now bedescribed with reference to the accompanying drawing, in which:

FIG. 1 shows a vacuum pump having a drag pumping mechanism and aregenerative pumping mechanism.

DETAILED DESCRIPTION

Referring to FIG. 1, a vacuum pump 10 is shown comprising a regenerativepumping mechanism 12 and a drag pumping mechanism 14. The regenerativepumping mechanism 12 comprises a generally disc-shaped member 16 mountedon an axial shaft 18 for rotation relative to a stator 20 of theregenerative pumping mechanism. The drag pumping mechanism 14 comprisinga generally cylindrical-shaped member 22 mounted at an outercircumferential portion 24 of the disc-shaped member 16 for rotationabout the axial shaft 18 relative to a stator of the drag pumpingmechanism. As shown in FIG. 1 a portion of the stator of the dragpumping mechanism is formed by stator 20 and another portion of thestator of the drag pumping mechanism is formed by a separate stator part26. Rotation of the disc-shaped member 16 and the cylindrical-shapedmember 22 by the axial drive shaft causes fluid to flow radiallyinwardly in series through the drag pumping mechanism and theregenerative pumping mechanism. The shaft is driven by a motor 28 andmay rotate at speeds of around 40,000 rpm.

The rotor disc-shaped member 16 has a plurality of rotor formations 30for pumping gas along channels 32. In order not to overcomplicate FIG. 1not all of the formations and channels have been labelled. As describedin more detail in the applicant's earlier applications WO2010/133866,WO2010/133867 and WO2010/133868, the contents of which are hereinincorporated by reference, the rotor formations 30 are formed by aseries of shaped recesses arranged in concentric circles at respectivecircumferences on the disc-shaped member 16. Three such circumferencesare shown in FIG. 1, however, greater or fewer numbers can be provideddepending on requirements. The concentric circles are aligned withrespective circumferential channels formed in the stator 20. The shapedrecesses 30 and the stator channels 32 form a gas flow path from aninlet to an outlet of the pumping mechanism so that when the rotor isrotated the shaped recesses generate a gas vortex which flows along theflow path. The stator channels 32 are circumferential throughout most oftheir extent but comprise a generally straight section for directing gasfrom one channel to a radially inner channel.

As shown in FIG. 1, the disc-shaped member 16 comprises rotor formations30 on opposing sides. Three arrays of rotor formations are located on anupper side of the disc 16 and three arrays of rotor formations arelocated on the lower side of the disc 16. The upper and lower rotorformations 30 are co-operable with channels 32 of complementary portionsof the stator 20. In FIG. 1, the channels 32 located above the disccooperate with the rotor formations on the upper side of the disc andthe channels 32 located below the disc cooperate with the rotorformations 30 on the lower surface of the disc. Therefore, theregenerative pumping mechanism 12 can pump fluid along parallel fluidflow paths on the opposing sides of the disc. The parallel flow pathsare shown schematically by arrows 34 pointing radially inwardly,although it will be appreciated that the flow paths are in realityconveyed circumferential around each channel 32 and from one channel tothe next inner channel. The parallel flow paths 34 of the regenerativepumping mechanism are symmetrical about a central plane P of the disc 16such that forces generated during pumping fluid along said parallel flowpaths are balanced in the regenerative pumping mechanism.

The generally cylindrical member 22 of the drag pumping mechanism 14cooperates with the stator 20, 26 to pump fluid along spiral channelsformed in either the cylindrical member or the stator. The generalarrangement of such a Holweck type pumping mechanism is known to theskilled person. In the arrangement shown in FIG. 1, the cylinder 22comprises a first, outer, cylindrical portion and a second, inner,cylindrical portion extending on opposing axial sides of the cylinder.The outer portion of the cylinder faces the inner portion of stator part26. The inner portion of the cylinder faces the outer portion of thestator 20. Fluid is pumped in parallel along flow paths shownschematically by arrows 36.

The generally cylindrical member is preferably made form a strong lightmaterial such as carbon fibre for reducing inertia about the axial shaftand reducing bending at the axial ends of the cylinder during rotationat speeds of around 40,000 rpm. It will be appreciated that the axialspacing between the rotor and the stator needs to be controlledaccurately in order to improve performance and therefore a reduction inbending of the cylinder reduces the required tolerance between the rotorand the stator.

Fluid enters the vacuum pump 10 through inlet 38 and is conveyed alongparallel pumping channels 36. The parallel flow paths of the dragpumping mechanism are symmetrical about the central plane P andtherefore unlike the earlier patent application, the pressure along theflow paths is generally equal one flow path with the other flow.Therefore, the forces on each side of the drag pumping mechanismbalance.

Fluid conveyed from the parallel flow paths of the drag pumpingmechanism 14 passes into respective flow paths 34 of the regenerativepumping mechanism 12. Flow paths terminate towards the radial centre ofthe vacuum pump proximate the axial shaft 18. Fluid conveyed along theflow path 34 above the disc 16 passes through bores 38 extending throughthe disc. Subsequently fluid passing along both parallel paths 34, 36 isexhausted through outlet 40 typically at atmospheric pressure.

An axial gas bearing arrangement is located at an inner circumference ofthe generally disc-shaped member 16 for maintaining an axial clearance Xbetween the disc-shaped member and the stator of the regenerativepumping mechanism. The axial gas bearing arrangement comprises first andsecond axial gas bearings 42, 44 on opposing sides of the generallydisc-shaped member. The axial gas bearing arrangement is described indetail in the present applicant's earlier application, however in theearlier application the gas bearing is located at an outer circumferenceof the disc-shaped member of the regenerative mechanism unlike thepresent application where it is located at an inner circumference. Inthe present application, the cylinder 22 of the drag pumping mechanismis located at an outer circumference of the disc 16 proximate the inlet38 and therefore at low pressure. The gas bearing 42, 44 is thereforelocated at the inner circumference since gas bearings require pressureclose to atmosphere to perform acceptably. In other respects, thepresent gas bearing is similar to that described in the earlierapplication and therefore will be described only briefly in thisapplication.

The axial gas bearings 42, 44 comprises a rotor part on the disc and astator part on the stator 20 located so that gas pumped along the flowpaths between the inlet and the outlet can pass between the two partsfor controlling the axial clearance X. The gas bearing is beneficialbecause it allows a small axial running clearance between rotor andstator which is necessary for reducing leakage and producing anefficient small pump. Typical axial clearances achievable in embodimentsof the invention are in the range of 10-30 μm and even 10-15 μm. Theaxial clearance between the rotor and stator of the regenerative pumpingmechanism is controlled by the gas bearing, whereas the radial clearancebetween the rotor and the stator of the drag pumping mechanism iscontrolled by ball bearings, described in more detail below.

In FIG. 1, the axial gas bearing are located on the flow paths betweenthe inlet 38 and the outlet 40 of the pump and therefore it is thepumped gas that forms the gas that supports loading in the bearings. Insome pumping applications the pumped gas may contain entrained dirt ordust which can become clogged in the axial gas bearings reducing theirefficiency. In a modification of the FIG. 1 arrangement, the gas whichhas been pumped along the flow paths 34, 36 is diverted towards theoutlet 40 prior to passing through the axial gas bearings 42, 44 toreduce clogging of the bearings by dirt or dust. The diversion paths maybe formed in the rotor and/or stator radially outward from the axial gasbearings. In this case, the axial gas bearings may use ambient gas inthe region of the shaft 18 which is typically at atmosphere forsupporting loading between the bearing surfaces.

Although an air bearing is able to produce small axial runningclearances, air bearings are not well suited to carrying relativelyheavy loads. Accordingly, in FIG. 1, the flow paths 34 of theregenerative pumping mechanism are symmetrical on each side of the disc16 relative to the central plane P extending orthogonally to the axialshaft 18. Additionally, the flow paths 36 are symmetrical between anupper portion of the cylinder 22 and a lower portion of the cylinder.Therefore, the forces generated during pumping are generally balanced tosuch an extent that the air bearing 28 can resist the relatively smalland transient loading applied to the bearing.

The gas bearings are located on the atmospheric side of the pumpingmechanisms where the pressure of pumped gas is greatest and typically atatmosphere. In FIG. 1, the atmospheric side of the pumping mechanisms isat a radially central part of the pump. In the applicant's earlierapplication, it is indicated that the gas bearing is preferably locatedat a radially outer part of the pump, where tangential velocity of therotating disc of the regenerative pumping mechanism is highest. Inaddition to the pressure of gas in the bearings, the tangential velocitybetween relatively rotating parts also contributes to the bearing forceswhich can be generated by the gas bearing. The location of the gasbearing at a radially central part of the pump, where tangentialvelocity of the disc 16 is lowest, is therefore counter-intuitive. Aconsequence of locating the gas bearing at radially outer part of therotating disc in the earlier application is that the low pressurestage(s) of the regenerative pumping mechanism are located at a centralpart of the mechanism whereas the high pressure stage(s) are located atan outer part of the mechanism. The applicant's have found however thatthe benefits arising from locating the gas bearing at an outer part ofthe pump are outweighed by the detrimental effect of locating the lowpressure stages towards the centre of the pump and the high pressurestages towards the outer part of the pump. In this regard, low pressure(or high vacuum) stages are required to be relatively large so that theyare able to pump a reduced amount of gas at low pressure. Conversely,the high pressure (low vacuum) stages are required to be small becauseotherwise they would encounter significant gas resistance. In theearlier design the smaller high pressure stages are located at an outerpart of the pump where they are rotated with the greatest tangentialspeed and this higher speed increases gas resistance. Similarly, the lowpressure stages are located towards the centre of the pump where theyare rotated at the lowest tangential speed and this lower speed reducesthe amount of gas that can be pumped or the compression ratio that canbe achieved at low pressure. In the present application, the location ofthe stages enhances rather than detracts from the requirements of thestages at high and low vacuum, since the larger rotor formations 30 ofthe high vacuum stage are driven at the greatest tangential speed andthe smaller rotor formations 30 of the low vacuum stage are driven atthe lowest tangential speed. Rotation of the gas bearings 42, 44 atlower speeds is found to produce acceptable bearing forces.

Ball bearings 46, 48 are provided for supporting the drive shaft 18 atend portions thereof. The ball bearings control the radial location ofthe disc 16 and the cylinder 22. The radial location of the discrelative to the stator is less critical for pumping performance than therelative axial location, since the rotor formations 30 may be slightlyradially misaligned with their respective channels 32 withoutsignificantly reducing performance. Therefore, even though the ballbearings require greater tolerances and do not fix the relative radiallocation as accurately as the gas bearing described above, the ballbearings are acceptable. Additionally, the radial location of thecylinder 22 relative to the stator parts is important for theperformance of the drag pumping mechanism 14 (typically having a radialspacing of between about 0.1 to 0.5 mm) however this relative locationis less critical than the relative axial location of the disc 16relative to the stator 20 (typically having an axial spacing of 10-20μm). A back-up bearing (not shown) may be provided in the event offailure of the gas bearing to prevent significant damage to the pumpoccurring due to a high speed collision of rotor and stator.

The present vacuum pump is configured to pump down from atmosphere tohigh vacuums in the region of 10-3 mbar. When pumping commences, thepressure at the inlet 38 is at atmosphere. The drag pumping mechanism 14is not efficient at these pressures and therefore pump down fromatmosphere is performed primarily by the regenerative pumping mechanism12. However, the larger upstream stages of the regenerative mechanismare arranged to operate at low pressures but when operating at highpressures they encounter significant resistance to rotation. In the pumpshown in FIG. 1, a blow-off pathway 50 is provided which can vent toatmosphere through a blow-off valve 52.

The blow-off valve 52 selectively opens the fluid path 50 betweenatmosphere and a stage of the regenerative pumping mechanism inward fromthe outer circumference for bypassing at least one inner pumping stagefrom the fluid flow path 34. The blow-off valve may be configured toopen the fluid path 50 at pressures above a predetermined pressure andclose the path at pressures below the predetermined pressure, preferablywith some hysteresis. The switch-over pressure can be determined bythose skilled in the art simply by operating the pump and monitoringcharacteristics such as rotational speed and motor power. The valve maybe operably connected to a suitable located pressure sensor for sensingpressure in the pump and initiating opening and closing of the valve.

Alternatively, the valve may be arranged mechanically to open and closeat the selected pressure. In the arrangement shown, upper and lowerbypass flow paths and bypass valve are provided for bypassing one ormore pumping stages of the flow paths on both sides of the disc 16. Bothupper and lower valves are operated substantially simultaneously forclosing and opening so that the flow paths are symmetrical on both sideof the disc and the forces balance.

1. A vacuum pump comprising: a regenerative pumping mechanism comprisinga generally disc-shaped member mounted on an axial shaft for rotationrelative to a stator of the regenerative pumping mechanism; and a dragpumping mechanism, the drag pumping mechanism comprising a generallycylindrical-shaped member mounted at an outer circumferential portion ofthe disc-shaped member for rotation about the axial shaft relative to astator of the drag pumping mechanism, rotation of the disc-shaped memberand the cylindrical-shaped member by the axial shaft causing fluid toflow radially inwardly in series through the drag pumping mechanism andthe regenerative pumping mechanism.
 2. The vacuum pump of claim 1,wherein the generally disc-shaped member comprises rotor formations onopposing sides of the generally disc-shaped member that are co-operablewith complementary portions of the stator of the regenerative pumpingmechanism for pumping fluid along parallel fluid flow paths on theopposing sides.
 3. The vacuum pump of claim 1, wherein the generallycylindrical-shaped member comprises first and second cylindricalportions extending on opposing axial sides of the generally disc-shapedmember co-operable with complementary portions of the stator of the dragpumping mechanism for pumping fluid along parallel fluid flow paths onthe first and second cylindrical portions.
 4. The vacuum pump of claim2, wherein the parallel flow paths of the regenerative pumping mechanismare symmetrical about a central plane of the generally disc-shapedmember, the parallel flow paths of the drag pumping mechanism aresymmetrical about the central plane, or both, such that forces generatedduring pumping fluid along said parallel flow paths are balanced in theregenerative pumping mechanism, the drag pumping mechanism, or both. 5.The vacuum pump of claim 4, further comprising an axial gas bearingarrangement located at an inner circumference of the generallydisc-shaped member for maintaining an axial clearance between thegenerally disc-shaped member and the stator of the regenerative pumpingmechanism.
 6. The vacuum pump of claim 4, wherein the axial gas bearingarrangement comprises a first axial gas bearing and a second axial gasbearing on opposite sides of the generally disc-shaped member.
 7. Thevacuum pump of claim 6, wherein fluid passing along the parallel fluidflow paths is conveyed through at least one of the first or second axialgas bearings towards the outlet of the vacuum pump.
 8. The vacuum pumpas claimed in of claim 6, wherein fluid passing along the parallel flowspaths is diverted along one or more diversions paths in at least one ofthe generally disc-shaped member or the stator of the regenerativepumping mechanism for by-passing the axial gas bearings.
 9. A The vacuumpump of claim 1, wherein the regenerative pumping mechanism comprises aplurality of pumping stages arranged at respective circumferences of thegenerally disc-shaped member and the rotor formations of the pumpingstages decrease in size from an outer circumference towards an innercircumference.
 10. The vacuum pump of claim 9, further comprising ablow-off valve for selectively opening a fluid path between a stage ofthe regenerative pumping mechanism inward from the outer circumferenceof the generally disc-shaped member and atmosphere for bypassing atleast one inner pumping stage from the fluid flow path.
 11. The vacuumpump of claim 1, wherein the generally cylindrical member comprisescarbon fiber for reducing inertia about the axial shaft.
 12. The vacuumpump of claim 2, wherein the generally cylindrical-shaped membercomprises first and second cylindrical portions extending on opposingaxial sides of the generally disc-shaped member co-operable withcomplementary portions of the stator of the drag pumping mechanism forpumping fluid along parallel fluid flow paths on the first and secondcylindrical portions.
 13. The vacuum pump of claim 3, wherein theparallel flow paths of the regenerative pumping mechanism aresymmetrical about a central plane of the generally disc-shaped member,the parallel flow paths of the drag pumping mechanism are symmetricalabout the central plane, or both, such that forces generated duringpumping fluid along said parallel flow paths are balanced in theregenerative pumping mechanism, the drag pumping mechanism, or both. 14.The vacuum pump of claim 13, further comprising an axial gas bearingarrangement located at an inner circumference of the generallydisc-shaped member for maintaining an axial clearance between thegenerally disc-shaped member and the stator of the regenerative pumpingmechanism.
 15. The vacuum pump of claim 13, wherein the axial gasbearing arrangement comprises a first axial gas bearing and a secondaxial gas bearing on opposite sides of the generally disc-shaped member.16. The vacuum pump of claim 15, wherein fluid passing along theparallel fluid flow paths is conveyed through at least one of the firstor second axial gas bearings towards the outlet of the vacuum pump. 17.The vacuum pump of claim 15, wherein fluid passing along the parallelflows paths is diverted along one or more diversions paths in at leastone of the generally disc-shaped member or the stator of theregenerative pumping mechanism for by-passing the axial gas bearings.18. The vacuum pump of claim 2, wherein the regenerative pumpingmechanism comprises a plurality of pumping stages arranged at respectivecircumferences of the generally disc-shaped member and rotor formationsof the pumping stages decrease in size from an outer circumferencetowards an inner circumference, further comprising a blow-off valve forselectively opening a fluid path between a stage of the regenerativepumping mechanism inward from the outer circumference of the generallydisc-shaped member and atmosphere for bypassing at least one innerpumping stage from the fluid flow path.
 19. The vacuum pump of claim 3,wherein the regenerative pumping mechanism comprises a plurality ofpumping stages arranged at respective circumferences of the generallydisc-shaped member and rotor formations of the pumping stages decreasein size from an outer circumference towards an inner circumference,further comprising a blow-off valve for selectively opening a fluid pathbetween a stage of the regenerative pumping mechanism inward from theouter circumference of the generally disc-shaped member and atmospherefor bypassing at least one inner pumping stage from the fluid flow path.20. The vacuum pump of claim 4, wherein the regenerative pumpingmechanism comprises a plurality of pumping stages arranged at respectivecircumferences of the generally disc-shaped member and rotor formationsof the pumping stages decrease in size from an outer circumferencetowards an inner circumference, further comprising a blow-off valve forselectively opening a fluid path between a stage of the regenerativepumping mechanism inward from the outer circumference of the generallydisc-shaped member and atmosphere for bypassing at least one innerpumping stage from the fluid flow path.